This invention relates to a method of forming a green tire. More particularly, the present invention relates to a method of forming a green tire which makes it possible to shape tire constituent portions with high dimensional accuracy while keeping high productivity by a strip winding method.
Pneumatic tires in general are produced by fitting a green tire shaped into substantially the same structure and size as those of complete tires and from an unvulcanized rubber into a mold and vulcanizing and molding the green tire. Tire constituent portions of such a green tire, such as a cap tread, a side tread, etc, are formed by winding a rubber sheet material having a shape similar to the cross-sectional shape of each tire constituent portion in a length corresponding to the tire peripheral length, and splicing both end portions of the rubber sheet material. In the pneumatic tires produced by this splicing method of the rubber sheet material, however, the splicing portion remains as a step and invites the problem of tire uniformity.
In contrast with such a splicing method, a so-called "strip winding method" is known which comprises the steps of extruding the unvulcanized rubber in a thin strip from an extruder, winding continuously a plurality of times the rubber strip round a tire intermediate body and shaping the overall sectional shape so integrated into the shape corresponding to that of the tire constituent portion. Since this strip winding method does not yield the splicing portion, a pneumatic tire having high uniformity can be obtained. Further, because a tire constituent portion having a predetermined size can be shaped by merely winding the rubber strip in layers, productivity is high. Moreover, because the strip winding method does not need expensive mouthpieces the splicing method need to prepare for each rubber sheet material, it can be said very advantageous from the aspect of the production cost.
Notwithstanding these advantages, the strip winding method is not free from the drawback that tire dimensional accuracy cannot be improved because an extrusion speed is difficult to control when the rubber strip is continuously extruded from an extruder. In other words, when the tire constituent portion is molded by winding the rubber strip in layers in this strip winding method, the peripheral speed increases as the outer peripheral diameter grows, and it is difficult to let the extrusion speed of the rubber strip of the extruder quickly follow the increase of this peripheral speed. Because the extrusion quantity gets unstable particularly at the start and end of extrusion, it is difficult to stably feed the rubber strip in a correct quantity in synchronism with the rotation of the building drum.